Login to the Apple website or via iTunes and update your credit card information. The problem could be an expired credit card, insufficient funds or detected fraudulent activity, to name a few. In any case, contact Apple.

It happened to me, too. I was about to go off on somebody, too...lol...until I saw the expiration date. The new card was atop of the kitchen counter still in the envelope that I had not yet opened....#smh.